The Mesopotamian calendar was primarily lunar, and the months began with the first sighting of the young, waxing crescent moon just after sunset on the western horizon. Consequently, each month was either 29 or 30 days long, and the year approximately 354 days.

Ritual celebrations, however, were aligned with the seasons, and thus solar in nature. Therefore, intercalary months were added when necessary to keep the lunar calendar in agreement with the seasonal schedule.

The practice of intercalation seems not to have been standardized until quite late. Possibly some time between 383 and 367 BC. Before then, a 13th month was added around every 3 years. But, again, this was not done with any particular regularity, or by any calculated method. Rather, the moon's phases were simply observed nightly. When the young crescent was sighted, it was reported to the palace, and the new month was officially announced. The decision as to whether or not an intercalary month was needed had only observational guidelines, examples of which exist in texts such as MUL.APIN.

The Mesopotamian calendar as a whole was, for quite some time, no more standardized than its intercalations. Month names and festivals varied from city to city. However, around the middle of the 2nd millennium BC, the Standard Mesopotamian calendar gained widespread acceptance, mostly for political and commercial reasons. The Standard Mesopotamian month names are as follows:

Nisannu

Ajaru

Simanu

Tamuzu

Abu

Ululu

Tashritu

Arahsamnu

Kissilimu

Tebetu

Shabatu

Addaru

It is worth noting that, because the New Year coincided with the vernal equinox, the Mesopotamian month began in the middle of one of our months. Thus:

Nisannu = March-April
Ajaru = April-May

and so forth. The equinoxes play a significant role in just about all of the Mesopotamian calendars, and often the year was seen as being a union of two six-month "equinox years", rather than one twelve-month expanse. A pronounced example of this will be shown in the calendar of Ur.

Because there are so many interesting variations prior to the Standard Mesopotamian calendar, I'd like to discuss a few of them in greater detail. So, from here you can continue on to check out those from: